So, last year I took my 10 yr old to US. Her mom and I went to the Amphur office and they had a letter for us to sign for just such a case. Also I had photo copies of her mom's Thai passport and DL.
No one asked for anything. The airline checking in didn't care, We went through the Thai passport control and he didn't ask for anything, at BKK.
I would expect your son being 15, they wouldn't ask either.